1. Field of the Invention
The invention relates to a method for manufacturing an optical waveguide comprising an optoelectronic integrated circuit (OEIC), an optical waveguide, an LSI for OEIC control, an IC and a high frequency electric circuit on one substrate and an optoelectronic hybrid substrate using the optical waveguide, more specifically to a manufacturing method of an optical waveguide using a siloxane polymer, and an optoelectronic hybrid substrate using the optical waveguide comprising a siloxane polymer.
2. Description of the Prior Art
In response to demand for information processing of higher speed in optical communication systems or computers, use of light in signal transmission between integrated circuit chips, between substrates on which integrated circuit chips are mounted, or between boards comprising a plurality of integrated circuit substrates, is studied. However, since processing of transmitted signals is conducted in electronic parts in the case where light is used for signal transmission, signal conversion between a light signal and an electric signal is necessary in such an optical information processing.
In a conversion area of a light signal and an electric signal, a light transmission path comprising an optical fiber or an optical waveguide, or an photoelectron conversion element such as a laser diode and a photodiode is used. Therefore, in a circuit for optical information processing, an optoelectronic integrated circuit (OEIC) comprising a combination of the light transmission path or the photoelectron conversion element, a photoelectric element and an electronic circuit, an optical integrated circuit (optical IC) for processing information only with a light wave, an integrated circuit for controlling an electronic element or processing an electric signal, such as an IC or an LSI, and a high frequency electric circuit for driving an electronic part at a high speed coexist.
Hitherto, a module assembled with individual optical parts and electronic parts has been used as a photoelectron conversion element. Further, a light source system comprising a gas laser as a light source, and an optical system comprising a prism, a lens and a mirror was used. Recently, a comapct type optical system comprising a laser diode, a photodiode, an optical fiber and a rod lens has been used. And now research and development have been conducted strenuously for the practical use of an optoelectronic hybrid substrate, comprising an OEIC or an optical IC, an optical waveguide, an integrated circuit such as an LSI for OEIC control, and a high frequency electric circuit on one substrate.
According to the optoelectronic hybrid substrate, owing to high density integration, reduction of electric power, higher speed, higher reliability, improvement of mass productivity, easier assembly, and improvement of the stability after assembly can be achieved. However,in order for realizing an optoelectronic hybrid substrate, technology for forming an optical waveguide on a ceramic substrate, or a substrate comprising a thin film circuit laminated on a ceramic electric circuit substrate is necessary.
In the optical waveguide, precise control on a refractive index is necessary for a single mode light transmission. And for forming an optical waveguide, in particular, processing of a core portion and control of a refractive index of a core portion and a clad portion. For example, a typical specification for a silica-based single mode waveguide includes a core size of 7 .mu.m.times.7 .mu.m, the difference between a refractive index of the core portion and that of a clad portion of about 0.25%.
As a method of forming an optical waveguide, a method of preparing a core portion by forming an optical thin film of an appropriate thickness and applying a pattern processing thereto is generally used. As a method of forming an optical thin film, known methods include a CVD method, a FHD (Flame Hydrolysis Deposition) method, a vacuum evaporation method, a sputtering method, and an SOG (spin on glass) method. Among these examples, an SOG method is drawing attention since the method allows a thin film formation at a comparatively low temperature in a short time at a low cost.
In a sol-gel method using alkoxy silane as a starting material, which is an SOG method, a substantially complete inorganic silica can be obtained. However, it is difficult to obtain a film having a thickness of a several .mu.m or more in the method due to a large volume shrinkage at the time of depositing a film to generate a crack. Therefore the method has a problem of difficulty in the application in the production of a silica-based single mode optical waveguide where a thickness of at least 15 .mu.m is required for the core and clad portions. Further, there is another problem of a large internal stress of an obtained film, which causes a birefringence.
On the other hand, use of organic materials such as polyimide, PMMA (polymethyl methacrylate) and polycarbonate in addition to silica as a material of an optoelectronic hybrid substrate is advocated. Although known for easy film formation and a low cost, these organic materials have problems such as a high transmission loss and a high loss at a portion connecting with an optical fiber due to a refractive index larger than a silica-based material. Besides, these material excluding polyimide have problems such as high water absorption, a low decomposition temperature and a low glass transition point.
In the case polyimide fluoride is used as an optical waveguide material, there is a problem that a Fresnel reflection loss at the time of connecting with a silica-based optical fiber becomes larger due to a large difference between a refractive index of polyimide fluoride, which is about 1.53 and a refractive index of a silica, which is about 1.44. Besides, there is another problem of difficulty in connecting with a silica-based single mode optical fiber due to a multi mode light transmission.
Japanese Unexamined Patent Publication JP-A 5-66301(1993) discloses an optical waveguide material comprising siloxane-containing polymer and an optical material using thereof, which allows easier control on a refractive index in combination with another element substituting a silicon element. The disclosure suggests a method of changing a refractive index of an unsubstituted polymer by partially substituting a tetravalent element except silicon or a trivalent element except rare earth elements for a silicon element in a siloxane-containing polymer in a substitution method for obtaining an aimed siloxane polymer by hydrolysis and polymerization of a silane chloride and a chloride of an element to be substituted, such as Al.
The method has a problem of difficulty in reaction control since highly reactive materials are mixed. Therefore there is a problem that a sophisticated technology and an expensive equipment are necessary for producing a stable mixture with an optional mixing ratio.
The Japanese Unexamined Patent Publication JP-A 6-109936 (1994) and the U.S. Pat. No. 5,062,680 disclose an optical waveguide comprising a deuterated or halogenated siloxane polymer. The siloxane-containing polymer allows easy processing and a low loss in the range of a visible light and an infrared light in producing an optical waveguide. However, the use of deuterium causes increase of the cost and the use of a halogen element, which is highly active, causes a problem in handling.
In general, various conditions are required for an optical waveguide, such as a smooth film surface of an optical waveguide with a surface roughness (Ra) of at least 1/10 or less of a light source wavelength to be used, which is preferable for achieving a low propagation loss in the formation of a core portion, optional setting of an internal refractive index in the production of an optical waveguide particularly in the production of a single mode optical waveguide, where a fine refractive index control is required, an excellent flatness after film formation regardless of the unevenness or surface roughness of a base substrate in the case where an optical waveguide is formed on a thin film circuit substrate, adherence with a base substrate, and a heat resistance in mounting an optical device such as a laser diode or a photodiode or an electronic device such as an LSI by soldering in the case of formation of an optoelectronic hybrid substrate. Therefore, in order to achieve an practical use of an optoelectronic hybrid substrate, an optical waveguide allowing easy control of a refractive index produced in an SOG method with a simple equipment, and a production method thereof have been called for.